Buggy code:
public interface RedBlackTreeSet < E , N extends ValuedRedBlackNode < E > > extends NavigableSet < E > { N getRoot ( ) { } N createNode ( E value ) { } void setRoot ( N root ) { } default N getNode ( Object value ) { } default N getEndNode ( boolean first ) { } @ Override default int size ( ) { } @ Override default boolean isEmpty ( ) { } @ Override default boolean contains ( Object o ) { } @ Override default Object [ ] toArray ( ) { } @ Override default < T > T [ ] toArray ( T [ ] a ) { } @ Override default boolean containsAll ( Collection < ? > c ) { } @ Override default boolean retainAll ( Collection < ? > c ) { } @ Override default Iterator < E > iterator ( ) { } default Iterator < E > iterator ( boolean forward , E start , boolean includeStart , E end , boolean includeEnd ) { } default Iterator < N > nodeIterator ( boolean forward , E start , boolean includeStart , E end , boolean includeEnd ) { N startNode ; N root = getRoot ( ) ; if ( root == null ) startNode = null ; else if ( start == null ) startNode = getEndNode ( forward ) ; else startNode = ( ( N ) ( root . findClosestValue ( start , true , includeStart ) ) ) ; return new Iterator < N > ( ) { private N theLastNode = null ; private N theNextNode = startNode ; @ Override public boolean hasNext ( ) { if ( ( theNextNode ) == null ) return false ; if ( end == null ) return true ; int compare = theNextNode . compare ( theNextNode . getValue ( ) , end ) ; if ( compare > 0 ) return false ; if ( ( compare == 0 ) && ( ! includeEnd ) ) return false ; return true ; } @ Override public N next ( ) { N nextNode = theNextNode ; if ( nextNode == null ) throw new NoSuchElementException ( ) ; theLastNode = nextNode ; <START_BUG> theNextNode = ( ( N ) ( nextNode . getClosest ( forward ) ) ) ; <END_BUG> return theLastNode ; } @ Override public void remove ( ) { if ( ( theLastNode ) == null ) throw new IllegalStateException ( "remove()<seq2seq4repair_space>may<seq2seq4repair_space>only<seq2seq4repair_space>be<seq2seq4repair_space>called<seq2seq4repair_space>after<seq2seq4repair_space>next()<seq2seq4repair_space>and<seq2seq4repair_space>only<seq2seq4repair_space>once<seq2seq4repair_space>after<seq2seq4repair_space>each<seq2seq4repair_space>call<seq2seq4repair_space>to<seq2seq4repair_space>next()" ) ; setRoot ( ( ( N ) ( theLastNode . delete ( ) ) ) ) ; theLastNode = null ; } } ; } @ Override default boolean add ( E value ) { } default void simpleAdd ( E value ) { } @ Override default boolean remove ( Object value ) { } @ Override default boolean addAll ( Collection < ? extends E > c ) { } @ Override default boolean removeAll ( Collection < ? > c ) { } @ Override default void clear ( ) { } @ Override default E first ( ) { } @ Override default E last ( ) { } default N getClosestNode ( E value , boolean lesser , boolean withEqual ) { } default E getClosest ( E value , boolean lesser , boolean withEqual ) { } @ Override default E lower ( E e ) { } @ Override default E floor ( E e ) { } @ Override default E ceiling ( E e ) { } @ Override default E higher ( E e ) { } @ Override default E pollFirst ( ) { } @ Override default E pollLast ( ) { } @ Override default NavigableSet < E > descendingSet ( ) { } @ Override default Iterator < E > descendingIterator ( ) { } default NavigableSet < E > getSubSet ( boolean reverse , E fromElement , boolean fromInclusive , E toElement , boolean toInclusive ) { } default < V > NavigableSet < V > getMappedSubSet ( Function < ? super E , V > outMap , Function < ? super V , E > inMap , boolean reverse , V fromElement , boolean fromInclusive , V toElement , boolean toInclusive ) { } @ Override default NavigableSet < E > subSet ( E fromElement , boolean fromInclusive , E toElement , boolean toInclusive ) { } @ Override default NavigableSet < E > headSet ( E toElement , boolean inclusive ) { } @ Override default NavigableSet < E > tailSet ( E fromElement , boolean inclusive ) { } @ Override default SortedSet < E > subSet ( E fromElement , E toElement ) { } @ Override default SortedSet < E > headSet ( E toElement ) { } @ Override default SortedSet < E > tailSet ( E fromElement ) { } default RedBlackTreeSet . NodeSet < E , N > nodes ( ) { } default RedBlackTreeSet . NodeSet < E , N > nodes ( boolean reverse , E fromElement , boolean fromInclusive , E toElement , boolean toInclusive ) { } static class MappedSubSet < E , V > implements NavigableSet < V > { private final RedBlackTreeSet < E , ? > theTreeSet ; private final Function < ? super E , V > theOutMap ; private final Function < ? super V , E > theInMap ; private final boolean isReversed ; private final V theMin ; private final boolean isMinInclusive ; private final V theMax ; private final boolean isMaxInclusive ; MappedSubSet ( RedBlackTreeSet < E , ? > tree , Function < ? super E , V > outMap , Function < ? super V , E > inMap , boolean reverse , V min , boolean includeMin , V max , boolean includeMax ) { }

Fix:
theNextNode = ( ( N ) ( nextNode . getClosest ( ( ! forward ) ) ) ) ;